Oversampling A/D (analog-to-digital) converters sample analog input signals at a rate Dfs, where fs is approximately twice the bandwidth of the filtered analog input signal and D is an integer multiplier. Dfs exceeds twice the bandwidth of the analog input signal. Oversampling converter configurations typically include an anti-alias filter, a sampler and modulator (quantizer) operating at the elevated sampling rate Dfs, and a digital filter. The digital filter, commonly called a decimator, generally provides low-pass filtering to suppress signals above fs/2, and a sample-rate reduction to lower the sample rate to the desired rate fs. One consequence of higher input sampling rates is that oversampling converters typically have less stringent anti-alias filter requirements than traditional converters. In addition, oversampling converters permit lower quantization noise power in the signal band, and hence improved signal-to-noise ratio compared to traditional converters.
A/D converters typically operate over a specified maximum and minimum input signal range. The maximum input signal may be referred to as the converter's full-scale input value. Under optimal operating conditions, if a full-scale input is applied to the converter, the converter will generally provide a full-scale output. In reality, however, the converter's actual output typically differs from the ideal result. The difference between the actual output and the ideal output is known as the full-scale error. An accurate full-scale converter has very small full-scale error.
An accurate full-scale oversampling converter has significant practical importance in such fields as data acquisition, test and measurement instrumentation, industrial control, etc. Accurate full-scale oversampling converters are preferred because they typically provide an accurate conversion result, exceptional unwanted signal rejection capabilities and have simplified anti-alias requirements.
A delta-sigma (DS) modulator generates a digital output signal whose DC average is generally a good estimate of a DC or low-frequency input signal VIN, divided by a reference voltage VREF. Nominally, A/D conversion can be performed in the range−VREF<VIN<+VREF. However, conversion accuracy generally deteriorates when the input signal VIN approaches −VREF or +VREF. As a result, one step towards designing accurate full-scale oversampling converters and other digital signal processing components involves obtaining controllable and accurate reduction in the amplitude of VIN. U.S. Pat. No. 6,140,950 depicts a delta-sigma (DS) modulator that requires N+M switched capacitors for scaling an input voltage by N/M.